1. Field of the Invention
The present invention relates to optical information reading apparatus for reading optical information, such as, a bar code, and more specifically, to optical information reading apparatus that can reduce a thickness of an optical system included therein and thus an overall thickness of the optical information reading apparatus.
2. Description of the Prior Art
Optical information reading apparatuses have been known, wherein a laser beam emitted from a light source is reflected by a rotating mirror to become a scanning light which is then applied via a pattern mirror onto an object to be read, such as, a label bearing an optical information, for instance, a bar code, so as to scan the optical information. In these systems light reflected from the read object is converged, via the pattern mirror and the rotating mirror, onto a light-receiving surface of a light-receiving sensor (photodiodes) which then converts the received light representative of an image of the scanned optical information into an electric signal depending on an intensity of the received light.
Japanese First (unexamined) Patent Publication No. 63-150775 shows one type of the optical systems used in the conventional optical information reading apparatuses. As shown in FIG. 9A, the optical system includes a ceiling mirror 105 arranged at a position obliquely above a rotating mirror 103 which is rotationally driven by an electric motor 101. A laser beam A emitted from a light source (not shown) is conducted via a through hole 107a formed in a condenser lens 107 to the ceiling mirror 105 where the laser beam A is reflected and incident onto the rotating mirror 103 from obliquely above. The incident laser beam A is reflected by the rotating mirror 103 in a horizontal direction to become a scanning light B which is then reflected by a pattern mirror 109 and applied or irradiated onto a bar code 113 via a reading window 111 of the optical information reading apparatus. A signal light C reflected on the bar code 113 is incident onto the rotating mirror 103 via the pattern mirror 109 and then reflected toward the ceiling mirror 105. The signal light C is further reflected by the ceiling mirror 105 and converged onto a light-receiving sensor 115 via the condenser lens 107.
However, in the shown optical system, since the signal light C reflected by the ceiling mirror 105 is a divergent light, the condenser lens 107 is required to be larger in size than the ceiling mirror 105, and an optical path to the light-receiving sensor 115 is required to be longer. Further, the condenser lens 107 is required to be perforate for the laser beam A to pass therethrough. Moreover, the light-receiving sensor 115 and the light source for the laser beam are required to be arranged close to each other.
On the other hand, the foregoing Japanese First Patent Publication No. 63-150775 also shows another type of conventional optical system. As shown in FIG. 9B, the optical system includes, instead of the ceiling mirror 105, a reflection hologram 120 having a light converging function. This structure is advantageous in, for example, that the condenser lens 107 in FIG. 9A is not required.
However, since the hologram has wavelength selectivity for its diffraction efficiency and wavelength dependency for its diffraction angle, a semiconductor laser which is effective for size reduction can not be used as a light source. Specifically, since the semiconductor laser changes a wavelength of emitting light depending on a variation in temperature thereof, when the wavelength of emitted light deviates from a design wavelength band, the diffraction efficiency of the hologram is largely reduced, and further, the light from the hologram is not precisely converged onto the light-receiving sensor 115 due to a change in the diffraction angle of the hologram. This causes insufficiency in intensity of the signal light which reaches the light-receiving sensor 115.